1. Field of the Invention
The present invention is directed to a gradient coil system for use in a magnetic resonance tomography (imaging) apparatus.
2.Description of the Prior Art
Conventional, permanently installed gradient coil systems occupy a not inconsiderable part of the interior of a magnet arrangement (called the xe2x80x9cwarm borexe2x80x9d given superconducting magnets). This shall be explained in greater detail with reference to FIG. 1. FIG. 1 shows the components of a conventional magnetic resonance tomography apparatus highly schematically. The basic magnetic field is generated with a magnet 1 in a toroidal coil arrangement that is usually a superconducting arrangement. Given superconducting basic field magnets, the coils (which are not shown in FIG. 1) are arranged in a cryostat. The basic field magnet has a hollow-cylindrical interior. A hollow-cylindrical gradient coil system 2 is arranged in this interior. A radio-frequency antenna is provided within the gradient coil system 2. The interior space remaining after the insulation of gradient coil system 2 and radio-frequency antenna 3 as well as after mounting cladding (not shown) is available as useful space. A patient 5 on a patient support 4 can be placed in this useful space. Certain minimum dimensions, particularly in view of the width, are required for the useful space in order to be able to examine obese patients at all or in order to be able examine patients in general without excessively serious constriction. The required inside diameter of the basic field magnet is thus established by the desired dimension of the useful space as well as by the radial expanse of the radio-frequency antenna 3 and the gradient coil system 2. The inside diameter of the basic field magnet 1, however, is a critically determining factor in the cost of the apparatus. If a larger diameter useful space is desired, not only must the toroidal coils be made larger but also, given superconducting coils, the cryostat as well be implemented with a large diameter. Due to the larger interior volume with a given magnetic field strength, more magnetic energy must also be employed. Given unchanging pre-requisites with respect to the required homogeneity in the examination space, finally, the length of the basic field magnet must also be increased together with the larger inside diameter. This is not only highly undesirable in view of costs, but also problems of claustrophobia for patients increase with greater length and the accessibility to the patient becomes poorer.
Of these systems installed in the examination space of the basic field magnet 1, the gradient coil arrangement has the highest space requirement.
It is therefore desirable to keep the outside diameter of the gradient coil system as small as possible in view of a predetermined acceptance space for the patient. Also operating in favor of a gradient coil having optimally small inside volume is that the sensitivity increases with smaller inside volume. xe2x80x9cSensitivityxe2x80x9d is the ratio of generated gradient field to current to be applied given the same number of turns. Given a small inside volume, lower demands are made of the gradient amplifier with a given gradient intensity.
In order to keep the volume enclosed by the gradient coil system smaller, German OS 196 53 449 discloses that the gradient coil be fashioned, not cylindrically as usual, but opened toward both sides. The especially critical shoulder space for the patient is thus expanded, so that the inside diameter of the basic field magnet can be made smaller.
Gradient coil systems known as local gradient coil systems are available for specific applications, that only accept a part of the body of a patient, for example the head. Such a local gradient coil system is disclosed, for example, in German OS 195 04 171, corresponding to U.S. Pat. No. 5,574,373. Due to its relatively slight inductance or, respectively, high sensitivity, such local gradient coils are particularly suitable for pulse sequences wherein gradients must be switched very fast, for example for the EPI (echo planar imaging) method.
The fields of application of such local coils, however, is limited.
An object of the present invention is to provide a gradient coil system with a small inside diameter and a wide range of application.
The above object is achieved in accordance with the principles of the present invention in a gradient coil system for a magnetic resonance tomography apparatus having two tesseral gradient coils (i.e. an x-gradient coil and a y-gradient coil) that are arranged on a substantially cylindrical surface, and a transverse z-gradient coil arrangement composed of a gradient coil pair (i.e., a first z-gradient coil and a second z-gradient coil), the transverse gradient coil projecting beyond the cylindrical surface at one end in the transverse direction, and with a spacing in the tesseral direction between the individual coils of the gradient coil pair of the transverse gradient coil arrangement. Due to the projection of the transverse gradient coil beyond the cylindrical surface and due to the distance (spacing) between the individual coils of the transverse gradient coil arrangement, an expanded acceptance possibility is created for a portion of the gradient coil system. Preferably, the spacing between the individual coils of the transverse gradient coil arrangement is designed to accept the shoulders of a patient, and the inside diameter of the cylindrical surface is designed to accept the head of a patient. The neck area thus also can be accepted in a head gradient coil.